Underwater objects towed by a towing surface vessel, such as, for example, towed linear acoustic antennas, may encounter partially submerged fishing lines. A towed linear acoustic antenna (also referred to as a “streamer antenna”) takes the form of a pipe of a length that is very great (potentially ranging up to several hundred meters long) with respect to its small diameter. Such an antenna is towed by a towing cable measuring several hundred meters and is intended to be submerged to fairly deep depths in the sea. Linear antennas are used for example for anti-submarine warfare or for oil prospecting. In general, a towed body (or towfish) is fixed to the end of the towing cable and is therefore towed at the same time as the antenna.
When detection operations are being carried out in a sea frequented by fishermen using fishing lines, it often happens that these fishing lines strike the towing cable, then slide along the cable before becoming trapped at the anchor point of the towing cable with or without a towed body. The fishing lines are then carried along by the towfish or the cable anchor and strike the antenna, under the effect of vortices. The various elements mounted on the fishing lines (hooks, shackles) may then damage the linear antenna.
Because the linear antenna is generally made from relatively soft elastic materials, the impact of these elements against the antenna often causes significant damage to the antenna and may also have the effect of damaging the towing cable at its anchor point. Furthermore, these lines caught up at this point present a risk of further accident when the cable and the antenna are raised back up to the surface through the fairleads.
A fishing-line-cutting device, mounted at the tail of the cable, has been proposed in patent FR 2 803 267 for cutting fishing lines that become caught in the device before they reach an underwater object towed by the cable. Such a device comprises an elongate blade holder fitted with a blade that can be retracted into a slot formed longitudinally and vertically in this blade holder, while rotating about a transverse thin pin situated at the rear of this blade holder. It also comprises an elongate U-shaped component provided with a straight upper branch and with a lower branch ending in a bill deviating from the axis of this component, these two branches delimiting an interior space in which the blade holder is placed so that it can be fixed to the upper arm.
Such a device can be incorporated only at the anchor point at the tail end of the towing cable for cutting fishing lines which become caught in the device, the cut lines then drifting off into the sea without becoming entangled with the towed object.
However, such a solution is suited only to a very specific towing system incorporating a towed body which is recovered on the outside of the surface vessel using an arm of the launch/recovery system belonging to the surface vessel. It is, on the one hand, attached permanently to the tip of the towed body by means of a special yoke at the interface with the towed body and, on the other hand, connected to the cable tail end hook by a strap. Such a fixing is used for each deployment, when the towed body is being connected to the towing cable.
This device cannot therefore be used for other configurations of towed systems. Furthermore, this device cannot be used directly on a linear antenna being towed on its own. It also has a bulk such that it cannot be used directly on a towed object that is intended to be brought back up to the surface vessel by passing through a specific fairlead, in the manner of a telepheric, before becoming accessible inside the ship. If fishing lines are caught, these may therefore either compromise the recovery of the towed object or lead to further accident (blockage or aggravated damage in the fairlead).